The Torah says that a tzaraas lesion can appear on an article of clothing - a garment made of wool or a garment made of linen - and also on the weaving threads themselves. In weaving there are vertical threads and horizontal threads woven into one another: "shesi" - the vertical threads, and "erev" - the horizontal threads. Whether the tzaraas appears on these threads or on those, and again the Torah lists: linen or wool, or leather.
The first opinion in the Mishnah - comparing wool to linen:
The Torah joins wool and linen together twice, in two verses, and according to the first opinion in the Mishnah the two are compared to one another for two different laws. Linen is naturally white, and beyond that it is not normally dyed another color. From here we learn that a wool garment too does not become tamei with tzaraas unless it is white and in its natural color, having not been dyed at all.
In addition, since that same verse also speaks of leather vessels, there is a parallel between leather and ordinary items made of wool or linen. Therefore we learn that leather garments too do not become tamei with tzaraas unless they are naturally white and their color has not been altered at all. Garments made of leather or of ordinary wool or linen fabric that have been dyed and are not naturally white cannot become tamei through lesions.
Lesions that appear on houses, by contrast, are not compared to linen at all, and therefore whether the house was painted or not, and even if it is not in its natural color, it can become tamei with tzaraas. This is the opinion of Rabbi Meir.
The opinion of Rabbi Yehudah:
Rabbi Yehudah says that processed hides have the same law as a house: they become tamei whether they are in their natural color or not, and they also need not be white. Rabbi Yehudah's reason is that although leather is written in the same verse together with garments made of wool or linen, the Torah does not say "and garments of leather," but rather "o b'chol meleches or" - "or in any article of leather" - and this word teaches that there is a distinction, and that the comparison between leather and the other garments is not complete. Leather indeed has a different law, the law of houses: regardless of its color, and whether it is in its natural color or not, it becomes tamei when a lesion appears on it.
The opinion of Rabbi Shimon:
Rabbi Shimon says that both sides can be satisfied together: both the word "o" ("or") in the verse, which teaches that the law of leather garments is different, and the Torah's comparison of leather to the ordinary garments made of wool and linen. Therefore, "hatzavua bidei Shamayim - mitamei, bidei adam - eino mitamei" - that which is colored by Heaven becomes tamei, and that which is colored by man does not become tamei: leather that is not white but that is in its natural color does become tamei through lesions, whereas leather that was dyed by hand by a person does not become tamei through lesions. The law of leather thus stands in the middle.
In summary:
A house: becomes tamei with tzaraas even if it was painted by a person.
Wool and linen: become tamei only if they are naturally white.
Leather according to Rabbi Shimon: becomes tamei even if it is colored, provided that this is its natural color.